In image compression such as JPEG (Joint Photographic Experts Group), an image is converted into YCbCr space and then compressed using DCT (Discrete Cosine Transform) or the like. In this process, human eyes have a visual perception characteristic that is sensitive to luminance variation but insensitive to chroma variation, and in some cases, utilizing such visual perception characteristic, a Cb plane and a Cr plane are subsampled (see PATENT LITERATURE #1).